1. Field of the Invention
The invention relates to a method for controlling a hydraulic rotary and feed drive for rotating and feeding tubes in a cold pilger rolling mill for producing tubes in which a rolling stand of the cold pilger rolling mill moves back and forth in the rolling direction via a crank drive, the rolls of the rolling stand being of a tapered design and rolling over the rolling stock with an alternating sense of rotation as the rolling stand moves back and forth, the rolling stand occasionally releasing the tube so that the tube can be rotated and fed to a new position by the hydraulic rotary and feed drive as a function of the position of the crankshaft of the rolling stand, and the rotation and feeding of the tube being performed by hydraulic motors controlled by a pressure medium and assigned to the rotary and feed drive members acting on the tube.
2. Description of the Related Art
In the 1960s, movements of rotary and feed drives for a tube during the execution of a cold pilger rolling method were represented hydromechanically. That is, cam plates were used to move control pistons of the valves of hydraulic motors. This mechanically complicated type of valve drive led to frequent failures. Furthermore, frequent pipe line breakages occurred because of very hard braking ramps.
In the cold pilger rolling mills of the 1990s, a very complicated servo-hydraulic path control has been used to track the rotary and feed movement otherwise produced purely mechanically with cam plate drives. This system operates reliably but has disadvantages with regard to production costs and maintenance. For example, the servo valves used in these systems are specialized products and therefore have long delivery times. Accordingly, when one of these valves becomes inoperable and requires replacement in the case of damage, the cold pilger rolling mill is inoperable or at least not optimally operable until the new valve can be delivered. To alleviate this concern, the servo valves may be stored as spare parts. However, the servo valves require regular cleaning of preservatives and renewing of the preservatives to maintain operability. Accordingly, a high maintenance cost is associated with storing the valves.
In addition, the complicated control path of these drives has the effect that it is difficult to locate faults when they occur. Well trained experts who can clarify whether there is a malfunction in the servo hydraulics, in the control or in the mechanism are required to trouble shoot the faults. Such highly qualified personnel are not available in many mills and must be summoned from, for example, a regional service center.